The invention relates in general to detection devices and in particular to a new and useful dosimeter for detection of noxious substances which are collected on a plaque-like substrate carrier, which is closed off against the outside by a diffusion membrane fitted to it.
A similar dosimeter is known from German OS 27 36 975,whereby a particulate adsorption material for collection of the noxious matter to be detected is filled in a recess housing. This plaque-like substrate carrier is covered by a membrane, which is permeable to the vapors being detected, which then penetrate into the adsorption material and can be detected by a subsequent assay method, such as the wet chemical method.
If such plaques are used with a colorimetric indicator, instead of the familiar adsorption material, it is important that the permeable membrane placed above the substrate carrier has a uniform distance over its entire region of coverage or fit closely on the substrate carrier in constant contact. Different distances also result in different diffusion path lengths, producing different color hues which, in actuality, do not originate in different concentrations of the substance being detected. Therefore, in covering the dosimeter, it is necessary to take the utmost care so that the membranes are uniformly tight and arranged at constant distance from the substrate carrier, which cannot always be achieved when the membranes have small dimensions and sensitivity. Even so, a careful covering may produce folds in the membrane. Fixation of the membrane at its margin produces the largest pressure on the substrate carrier in this place, which falls off steadily toward the center of the membrane and thus produces a pressure gradient, which leads to a varying pressure force on the substrate carrier over the entire stretch of the membrane. This also results in a varying diffusion behavior of the gas being detected from the outside into the substrate carrier, apart from the fact that the excessive fitting pressure and stretching of the membrane produces a locally varying diffusion behavior of the gas within the membrane.